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In the morning, microbes found on various lines of Hong Kong’s rail network are characteristic of the neighbourhood each line serves. But during the day, the flora from each line mix, and by the evening, the network’s microbiome reflects the entire city.

A team led by Gianni Panagiotou at the University of Hong Kong sampled the bacteria and yeast on the hands of six riders for eight lines of Hong Kong’s Mass Transit Railway System. On three test runs per line, riders held handrails and touched surfaces for 30 minutes during the morning rush hour and again during the evening rush hour.

The team found that both microbes and antibiotic-resistance genes mingled freely in the system. The sampling ﻿also showed that the sole line linking Hong Kong with mainland China hosts high levels of genes associated with antibiotic resistance, suggesting that this rail segment may act as a potential source of such genes to other lines over the course of the day.

A common air pollutant killed far fewer people in China in 2015 than it did in 2005, thanks in part to rural residents cutting back on the use of fuels such as wood and straw for cooking.

In 2005, China’s central government began setting restrictions on emissions of fine particulate matter — a mixture of microscopic particles that can be inhaled deep into the lungs — by vehicles, power plants and factories. To assess the campaign’s impact, Shuxiao Wang at Tsinghua University in Beijing, Kirk Smith at the University of California, Berkeley, and their colleagues modelled the Chinese population’s exposure to fine particulate matter.

The researchers found that deaths related to this form of pollution fell by 29% between 2005 and 2015. Surprisingly, government restrictions accounted for only a fraction of this decrease. Eighty per cent of the drop was spurred by a reduction in people’s use of wood and other dirty fuels for cooking and heating, as rural people either migrated to cities or gained enough income to buy cleaner fuels.

Millions of large, regularly spaced dirt mounds dot a Brazilian region roughly the size of Great Britain — and all because of termites.

The forests of northeastern Brazil boast some 200 million earthen mounds, each between two and four metres tall, roughly nine metres across and up to 4,000 years old. Oddly, each hill is spaced about 20 metres from any other hill in any direction.

Curious about these knolls, Stephen Martin at the University of Salford in Manchester, UK, and his colleagues examined satellite images of the area, as well as cross-sections of hundreds of the hills. Most were solid dirt. But those built more recently contained a tunnel leading to underground channels excavated by the subterranean termite Syntermes dirus.

The researchers suggest that the mounds are accumulated dirt that termites discarded as they dug their underground highways. The insects dumped the dirt at many evenly spaced locations, minimizing the time taken to reach a disposal site from any part of the network.

People tend to shower more efficiently if they see real-time data about their energy usage as they scrub.

Verena Tiefenbeck at the Swiss Federal Institute of Technology in Zurich and her colleagues equipped rooms at six hotels with a shower meter. Sixty per cent of the meters displayed real-time information about water and energy consumption, along with an animation of a polar bear on a melting ice floe. The remaining 40% of the meters displayed only the water temperature.

None of the hotel guests knew about the experiment, nor were they responsible for the water bills. Even so, guests whose meters provided instantaneous feedback used 11.4% less energy, on average, than those who did not receive this information, suggesting that such measures can cut the public’﻿s consumption of energy and other resources.

Like the caves of ogres in fairy tales, the nests of the Formica archboldi ant are littered with the ‘skulls’ of their prey: ants in the Odontomachus category. But how the head-hunting ants kill their victims has been a mystery. Odontomachus ants are larger than their predators, and seemingly fiercer; their common name, trap-jaw ants, alludes to their wickedly effective spring-loaded jaws.

Now, Adrian Smith at the North Carolina Museum of Natural Sciences in Raleigh has discovered how F. archboldi take down trap-jaws. Smith collected ants of both species from around Florida and analysed the chemical waxes that coat their bodies. He also filmed combat between the species to try to work out the attack strategies used by the skull-collecting ants.

He found that the chemical odours of the two species are nearly identical — a tactic that, he suggests, might allow F. archboldi to disguise itself among its prey. The attacking ants then paralyse their victims using a precisely delivered stream of formic acid.

A synthetic molecule can snatch a medically important sugar from blood, and might lead to new developments in diabetes therapy.

Many researchers have tried to build molecular receptors that can link to sugars, especially glucose, and extract these sugars from blood. But certain components of a glucose molecule are highly attractive to water. As a result, synthetic receptors in the blood often lose a tug-of-war against water molecules over a bit of glucose.

To win the battle, Anthony Davis at the University of Bristol, UK, and his colleagues synthesized a receptor that mimics sugar-scavenging proteins found in the body. The researchers added chemical groups to the receptor that made it more attractive to glucose and enabled it to grip sugars more tightly than other designs do.

The team’s version is nearly 100 times more attractive to glucose in water — and only slightly less than this in blood — than are previous versions of such receptors. The receptor captured glucose and similar sugars without grabbing unintended molecules, an efficacy that could make the design useful for applications such as blood-glucose monitors.

Surprised astronomers have realized that a promising object of study was just a trick of the light.

Planetary nebulae are glowing clouds of gas that are thrown off by a very hot, dying star. ‘Bipolar’ planetary nebulae feature two gas lobes on either end of a central star. They are thought to be formed by the death of a star involved in a pairing called a binary system.

Henri Boffin at the European Southern Observatory in Garching, Germany, and his colleagues trained a telescope on a bipolar planetary nebula roughly 4,000 parsecs from Earth. Initial observations of the nebula’s centre revealed a binary system — the potential parent system of the nebula and so a tantalizing target for study.

But further analysis showed that the stars in the binary system were healthy, not in their death throes. The team concluded that the binary is probably located a few thousand parsecs behind the nebula and thus unrelated.

The existence of this chance alignment means that stars located at the centre of planetary nebulae cannot always be assumed to be associated with them, the authors write.

Nearly a kilometre beneath Greenland’s ice sheet lies a buried impact crater, left by the crash of an enormous meteorite. The crater is probably among the 25 biggest known on Earth, and is the first found beneath an ice sheet.

Kurt Kjær at the University of Copenhagen and his colleagues sifted through radar data collected by aeroplanes that flew over Greenland to study its ice. They spotted a large depression, 31 kilometres across, buried beneath Hiawatha Glacier in northwestern Greenland.

At the edge of the ice sheet, outside the crater’s rim, the researchers discovered quartz grains that had apparently been washed downstream in a river of glacial runoff. The grains’ internal structure marked them as ‘shocked’ quartz, a variant formed through extremely high pressure and often associated with a meteorite impact.

The meteorite that formed the crater probably hit in the past 2.5 million years and might have stretched up to 1.5 kilometres across. The impact could have had major environmental consequences, such as temporarily changing the region’s climate.

The ancestors of the impressive Komodo dragon (Varanus komodoensis) must once have travelled long distances; otherwise, they could never have spread across the Indonesian islands where they live today. To see whether the lizards still have that wanderlust, Tim Jessop at Deakin University in Waurn Ponds, Australia, and his colleagues braved the lizards’ venomous bites to capture and microchip more than 1,000 individuals of varying sizes, allowing the researchers to record each animal’s movements over a period of up to a decade. The team also physically moved seven large, old dragons from one valley or island to another to see whether they would find their way home.

The results? Whatever peregrinations their ancestors might have undertaken, contemporary Komodo dragons almost never leave their home valleys, even if this means facing limited food supplies or an increased chance of inbreeding. If taken from their homes, they will return, unless an ocean stands in their way.

A large-scale trial has found no evidence that two popular supplements reduce the risk of cancer or the combined risk for a trio of cardiovascular problems.

JoAnn Manson at Brigham and Women’s Hospital in Boston, Massachusetts, and her colleagues recruited more than 25,000 healthy men and women in their fifties or older for a trial examining the effects of fish oil and vitamin D supplements. Some participants took both, others took only one type and the remaining participants took two placebos.

After an average of 5.3 years in the trial, participants who had taken fish oil had essentially the same likelihood of cancer as people who hadn’t. Compared with the placebo group, the fish-oil group had a lower rate of heart attack but the same rate of total cardiovascular events, a category that included heart attacks, strokes and death from cardiovascular disease.

Obesity can fuel tumour growth by suppressing the immune system’s response to cancer — but it could also render tumours particularly sensitive to certain cancer drugs.

Obesity increases the risk of some cancers, but its impact on immune responses to tumours has been unclear. William Murphy at the University of California, Davis, School of Medicine and his colleagues found that immune cells called T cells are more dysfunctional in obese mice and humans than in their non-obese counterparts.

Compared with T cells in thin mice, T cells in obese mice had higher levels of a protein called PD-1, which suppresses immune responses. The researchers found that one link to PD-1 levels is the hormone leptin, which helps to regulate appetite; treating leptin-deficient mice with leptin sped tumour growth and increased PD-1 levels.

The team also showed that tumours in obese mice and humans might be more vulnerable to therapies that inhibit PD-1. But elevated PD-1 expression in obese people could be one of many factors responsible for this effect, the authors say.

An atom has been caught spitting out a fundamental nuclear particle in fewer than 18 nanoseconds, setting a speed record for a particular type of radioactive decay.

Many radioactive elements, including tellurium-104, emit ‘alpha particles’, which consist of two protons and two neutrons. Nuclear theory predicts that tellurium-104 should be extremely quick to emit an alpha particle, because doing so creates tin-100, which has an especially stable nuclear configuration. But tellurium-104’s decay to tin-100 is too rapid for current methods to observe directly.

Kalle Auranen at the Argonne National Laboratory in Lemont, Illinois, and his colleagues produced tiny amounts of the element xenon-108 and isolated them using the Fragment Mass Analyzer at Argonne’s ATLAS accelerator. They then watched for the emission of two alpha particles in succession, as xenon decayed first to tellurium-104 and then to tin-100.

The team determined that tellurium-104’s half-life is under 18 nanoseconds. The atom’s brief life span lends credence to models predicting that heavy nuclei composed of equal numbers of protons and neutrons should experience enhanced nuclear forces, leading to swift ejection of alpha particles.

Non-rechargeable batteries that rely on a reaction between aluminium and atmospheric oxygen can hold much more energy than their conventional lithium–ion counterparts. A new tweak to the design of such batteries might overcome their greatest limitation: a short shelf life.

When an aluminium–air battery is not in use, its electrodes corrode, allowing unwanted electric discharge that shortens the battery’s lifespan. Brandon Hopkins at the Massachusetts Institute of Technology in Cambridge and his colleagues developed an aluminium–air battery that employs a conventional electrolyte while the battery is in use but, during storage, replaces the electrolyte with oil.

The battery achieves an energy-storage capacity of almost 900 watt-hours per kilogram — a capacity that is comparable with that of other aluminium–air battery designs — while reducing corrosion by 10,000-fold compared with other aluminium–air batteries. The authors suggest that such a battery could be used in long-range drones and for off-grid electricity generation.

Researchers have created models of human-heart tissue that beat just like the heart’s upper chambers.

Most artificially produced heart-muscle cells share characteristics with cells from the heart’s lower chambers, or ventricles. Thomas Eschenhagen at the University Medical Center Hamburg-Eppendorf in Germany and his colleagues sought to make cells similar to those of the organ’s upper chambers, known as the atria.

The group started with cells that had been reprogrammed into an embryonic-like state, and transformed them into heart-muscle cells. The researchers added retinoic acid, which is a derivative of vitamin A, to nudge the cells towards development into atrial-like tissue. The cells were then cultured in a fibrous, protein-based gel that allowed the tissue to grow in three dimensions.

The resulting 3D structures mimicked atrial heart muscle in the force of their contractions, their reaction to drugs and other properties. The scientists say that the beating structures could serve as models for drug development and for studies of how the heart functions.

A cache of human bones confirms historical accounts that the ancient Celts decapitated their enemies and embalmed the heads for public display.

Greek historians Diodorus and Strabo both wrote that Celtic warriors preserved the heads of their foes with cedar oil. To test this claim, Réjane Roure at the Paul Valéry University of Montpellier in France and her colleagues examined bits of skull excavated at Le Cailar, a fortified Celtic settlement in southern France. The skull fragments, which dated to the third century BC, were found with metal weapons and other artefacts in a large display area inside the settlement’s walls.

Chemical analysis revealed signatures of resin and plant oils on the skull fragments, which bore cut marks suggesting that the brains had been removed. The embalming mixture had “anti-odour properties”, the authors write, and would have slowed bacterial decay of the tissue.

It takes approximately the same amount of energy to mine a dollar’s worth of cryptocurrency as it does to mine a dollar’s worth of certain metals.

Cryptocurrencies such as Bitcoin have raised environmental concerns because ‘mining’ virtual coins requires energy-intensive computer calculations. But quantifying the environmental impact of cryptocurrency production is difficult.

Independent researchers Max Krause and Thabet Tolaymat calculated that it takes about 17 megajoules of computer power to generate US$1 in Bitcoin, even when the energy used for peripheral activities, such as cooling computers, is not factored in. By comparison, it takes 5 megajoules to mine US$1 in gold and 7 megajoules to mine an equivalent value of platinum. Aluminium-mining energy requirements, at 122 megajoules, topped the researchers’ list.

Over 30 months from 2016 to 2018, the authors report, Bitcoin mining produced an estimated 3 million to 13 million tonnes of carbon dioxide. That range does not account for mine operation and maintenance; even so, it is roughly equivalent to the range of carbon dioxide produced by about 1 million cars, although it is still less than 0.01% of global emissions.

The blanket of snow that shrouds diverse landscapes across Europe during the colder months is thinning rapidly, thanks to climate change.

Gerard van der Schrier at the Royal Netherlands Meteorological Institute in De Bilt and his colleagues analysed more than six decades of data on snow cover and climate from thousands of weather stations throughout Europe. They found that, with the exception of a few localized extreme cold spots, mean snow depth has decreased markedly since 1951 across the continent, at about 12% per decade. The depth of ‘extreme’ coverage — which cripples local infrastructure — decreased at a slightly slower pace.

The observed decline, which accelerated after the 1980s, is the result of a combination of rising temperatures and the effects of climate change on precipitation. Waning snow cover may lower the availability of fresh water during the spring melt period, the authors note.

A high-resolution satellite can detect signs of whales splashing and pooping in the oceans — and can even pick out a humpback whale’s long flippers.

Previous studies have used satellite data to count whales, but the WorldView-3 satellite, which was launched in 2014, captures images in much higher resolution than its predecessors. Hannah Cubaynes and her colleagues at the British Antarctic Survey in Cambridge, UK, searched WorldView-3 images of waters off the coasts of Italy, Mexico, Hawaii and Argentina for fin whales (Balaenoptera physalus), grey whales (Eschrichtius robustus), humpback whales (Megaptera novaeangliae) and southern right whales (Eubalaena australis), respectively.

Body shape and even the size of the creatures’ flippers and tail fins were visible in some photos, as were whale faeces and spray from the animals’ blowholes. The team found that grey whales and fin whales appear paler in colour than the water, making it easier to confidently identify them than the other species.

Satellite data could be used to develop an automated system for detecting and monitoring whales, the authors suggest.

Ancient Mongolians picked up a fondness for milk from their neighbours, evidence from human skeletons suggests.

Archaeological sites show that after about 1500 BC, the people of what is now Mongolia kept livestock. To learn whether those were dairy animals, Choongwon Jeong and Christina Warinner at the Max Planck Institute for the Science of Human History in Jena, Germany, and their colleagues analysed human skeletons buried in northern Mongolia and dated to 1380–975 BC. Proteins on some of the individuals’ teeth showed that they had consumed either milk or milk products from sheep, goats and animals related to dairy cattle.

During the period of the skeletons’ burial, people known as the western steppe herders had spread across Eurasia to the region just west of Mongolia. But the buried Mongolians’ DNA shows little genetic influence from these people.

This suggests that, rather than being replaced by the influx of western strangers, Mongolian hunter-gatherers took lessons from them in milking and dairy husbandry — lessons that would later be parlayed into the dairy-based Mongol empire.

A pig’s eyeball is injected with ‘micropropellers’, which are coated with a slippery substance inspired by a compound found in the throat of some carnivorous plants. Credit: Micro, Nano & Molecular Systems Lab, Max Planck Institute for Intelligent Systems

Tiny robots swim through the eye when propelled by a magnetic field — a technology that could one day be used to deliver drugs to currently inaccessible tissues.

Treatments for the retina, which lies at the back of the eye, usually involve the administration of drugs to the front of the eye. But as medicine diffuses to the retina, it can become less effective.

Tian Qiu and Peer Fischer at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, and their colleagues created drug-delivery vehicles in the form of microscopic glass corkscrews. The researchers deposited iron on the devices, and then coated each tiny screw with a slippery liquid designed to ease its path through the mesh of collagen inside the eye.

After injecting the 2-micrometre-long corkscrews into a pig’s eyeball, the team used a rotating magnetic field to propel the robots to a targeted region on the retina. The robots reached the back of the eye in one-tenth of the time it would have taken for a drug molecule to diffuse to the same location. The authors say that the devices could allow for faster drug delivery with fewer side effects than those of current methods.

A dark Galactic cloud similar to the Coalsack nebula (central black blob above) has been seen for the first time in the act of generating molecular hydrogen. Credit: ESO/Digitized Sky Survey 2/Davide De Martin

A chilly cloud of molecular gas in the Milky Way is giving astronomers a rare look at one of the earliest steps in star formation.

The smallest, most fundamental molecules in the Universe are created when two hydrogen atoms bond to form hydrogen molecules (H2). This process usually takes place in cold, dark clouds. But the molecule’s formation is rarely observed, because it’s hard to distinguish atomic and molecular hydrogen from other types of molecules and from each other.

Pei Zuo and Di Li at the National Astronomical Observatories of the Chinese Academy of Sciences in Beijing and their colleagues used the Arecibo radio telescope in Puerto Rico to observe dark clouds. The researchers found that one cloud had an outer ‘shell’ of atomic hydrogen that was being converted into molecular hydrogen — the first such detection of a dark cloud’s birth.

Further analysis of the rate of H2 formation suggested that the cloud is roughly six million years old. This finding could help to constrain models of star, planet and galaxy formation, the authors write.